. the state has recorded nearly 900 cases. that's 45% of all west nile cases in the united states. aerial pesticide spraying last month in and around dallas dramatically reduced the mosquito population. in july, half of the mosquitoes tested positive for west nile, now it's just 5%. but doctors believe the number of reported cases will increase because weeks can pass before people infected with west nile develop serious symptoms. dr. beth bell of the centers for disease control and prevention expects the outbreak to last until october. >> oh, it takes a little time for people after they get sick to go to the doctor, to get the blood test. what we're counting are cases of illness which occurred several weeks ago. >> and that's not the total number of people who may be sick from west nile virus. the cdc says hundreds more people probably have become ill but just never went to a doctor. anna werner, cbs news, dallas. >>> quick break now on a thursday morning. when we come back, central american shakeup. a major earthquake rattles costa rica, sending people running. this is the morning news.